liquid hammering on pumped liquid line R717

[VEYSEL DURLU]

Hello everyone.

i have liquid hammering problem in two large ammonia refrigeration plants.
System is pump recirculation with hermetic pumps.
The hammering happens on the liquid lines and after pump stops. When continiously running no problem.
Lines are -45 degree C , relatively long 100-150 m,
DN 65 and DN 80 size.

Has anyone got an idea.

[US Iceman]

Does the noise occur only once when the pump shuts down?

[VEYSEL DURLU]

I think the last comments i made is wrong . Since it is -45 liquid and 3 bar pump pressure on it it is like subcooled liquid. so no gas bubbles occurs.

[VEYSEL DURLU]

It does not happen only once. It continiues several minutes randomly

[US Iceman]

Why is the pump shutting off?

And, does it makes these noises randomly for a few minutes every time the pump does shut off?

[VEYSEL DURLU]

pump shuts off automatically when no cooling required. That means when all evaporators fed by this pump stops.
Yes, it happens every time it stops

[US Iceman]

Do the pumps pump liquid up to a high elevation to the evaporators?

The reasons for the questions, is I suspect this; once the pumps stop, the liquid tries to flow backwards through the liquid line. When the check valve closes on the refrigerant pump discharge (after the pump slows down or stops), the energy created by the backward flowing liquid causes the hammering effect.

[VEYSEL DURLU]

yes it is quite high appr 12 m.
sorry but i did not quite understand how come it can flow backward when the pipe is full of liquid and non return valve closes very fast.
I know that non return valve closes properly.
I suspect gas bubbles comes out due to heat gain along the piping after pump stops and these bubbles trapped somewhere ???

[US Iceman]

When the pump motor is deenergized the pump no longer creates pressure to flow to occur. So, as the liquid flow begins to slow down and then reverse the check (non-return) valve closes quickly.

With 12 meters of liquid height this also helps to push the liquid back down to the non-return valve so it closes quickly. When the liquid flowing in reverse hits the valve a shock wave is created and the noise is generated.

This is similar to a quick closing solenoid valve. When they close with a relatively high velocity liquid flowing through them the pipes can jump and make noise also.

These are the easist and most common issues I can think of.

[Sergei]

I think that this is a bubble. If liquid line doesn't have flow, bubble can be created by heat gained from outside. When pump is running, this bubble is locked by pump pressure. As soon as pump off and pressure down, this bubble will start moving in one direction and liquid will move in opposite direction and hummer will occur. Sometimes you will have hummer, when pump starts up. Keep at least one pump running to avoid this hummer.

[nh3wizard]

Why would you want to keep a pump running when its not needed? Seems like a waste of energy.

[Sergei]

Why would you want to keep a pump running when its not needed? Seems like a waste of energy.

I think that should be balance between safety and energy savings.
When we stop liquid pump, hummer can happen but usually it is not significant. Than liquid gradually drain from liquid line into vessel. Than pump start up and liquid will fly into empty liquid line and suddenly stop at the end of this line. This is major hummer. When liquid line is empty, liquid pump should start up with closed discharge valve. Than this valve should be open gradually.
Cycling the liquid pump will save less than 1 % of total power consumption. There are many points where we can save 10-20 % of energy consumption without any compromise to the safety

[nh3wizard]

I agree "Safety First", but 1% here and 1% there eventually will add up

[Sergei]

Why don't save 10-20% without compromise to the safety? I believe in safe, smart energy savings.

[US Iceman]

There are many points where we can save 10-20 % of energy consumption without any compromise to the safety.

This is true. There are a lot of opportunities to save energy in a refrigeration system, if they are designed properly for the operating conditions they will encounter.

The last point underlined above could be identified with the liquid hammering problem. My guess is the system was designed to operate at full load without any review of what happens when the system is shut off (or operating at part-load or started up for that matter).

[nh3wizard]

As far as "Hummers" go, I don't complain when I get them

[Sergei]

Why do all evaporators shut off? Design is one side of this issue, another side is proper operation. I found that significant fluctuation of refrigeration load on the plant can happen when we operate too many coils or suction pressure too low.

[US Iceman]

As far as "Hummers" go, I don't complain when I get them

I like mine!

[VEYSEL DURLU]

This is true. There are a lot of opportunities to save energy in a refrigeration system, if they are designed properly for the operating conditions they will encounter.

The last point underlined above could be identified with the liquid hammering problem. My guess is the system was designed to operate at full load without any review of what happens when the system is shut off (or operating at part-load or started up for that matter).

If anyone can tell me what is the design criteria, features with respect to liquid hammering in the discussed case.
For me this is a very important safety issue on refrigeration plant.

[Magoo]

your problem possibly stems from the fact that there could be a solenoid valve and by-pass relief after the pump. When system stops and sub cooled liquid warms up the relief bypass does its purpose of avoiding hydraulicing liquid line, try lowering relief valve setting just above supply pressure. This will ease the hammering.
magoo

[VEYSEL DURLU]

your problem possibly stems from the fact that there could be a solenoid valve and by-pass relief after the pump. When system stops and sub cooled liquid warms up the relief bypass does its purpose of avoiding hydraulicing liquid line, try lowering relief valve setting just above supply pressure. This will ease the hammering.
magoo

Your right there is a by pass line to seperator with PM+CVPP to get e constant pump pressure. This caused hammering at the first pace and it is solved.
The continiuing hammering comes from the liquid line to evaporators